[vm/ffi] NativeFieldWrapperClass1 FfiNative methods should check for nullptr

[dcharkes]

class Scene extends NativeFieldWrapperClass1 {
  @FfiNative<Void Function(Pointer<Void>)>('Scene::dispose')
  external void dispose();
}

NativeFieldWrapperClasses often share their identity between Dart and C++.

The first, and in this case only native field represents the C++ identity. The Dart object itself has the Dart identity.

If the C++ part has already been released/disposed/cleared, the native field is usually set to nullptr (unboxed to 0).

If the native field was never set (it is Dart null), trying to call dispose() will throw. However, if the native field has been cleared to nullptr, a C call is made with C++ this as nullptr usually leading to a segfault in the native code.

We should check that the native field value is not nullptr in addition to checking it for null.

We should only do this check for FfiNative instance methods on NativeFieldWrapperClasses, and only for the automatically converted receiver. It is perfectly fine to pass nullptr to a Foo* argument to any argument of a top level function or to any argument (except for the receiver) of a C++ method.

Flutter issue:

• Access disposed canvas in VM causes segfault flutter/flutter#109180